The present invention relates to an inexpensive, efficient, in-line process for incorporation of useful additives during the production of corrugated board. The process is particularly useful in producing corrugated board have excellent rigid-when-wet and rigid-when-humid strengths. Additionally, flame retardants, fire retardants, fungicides, mold inhibitors, insecticides, and the like are readily incorporated into the board in an in-line process. The process comprises applying a collapsible foam of a dispersion or solution of the desired additive to the corrugated medium, and desirably the liners, during in-line assembly of the corrugated board from the corrugated medium and facing liner components.
In the manufacture of corrugated paperboard, it is the practice to form the corrugated assembly by sandwiching a corrugated medium between a pair of spaced-apart linerboards in a corrugating machine. The corrugated paperboard is then cut into sections or into blanks for storage and scored to form containers for shipment of goods therein.
While conventional corrugated paperboard materials have enjoyed a high degree of commercial success, this type of material is normally relatively unsatisfactory when exposed to high-humidity or wet conditions, such as those encountered when shipping produce, dressed poultry, or the like, packed in ice. The exposure of the corrugated board to water and water vapor rapidly deteriorates the corrugated board so that it will completely collapse in a relatively short period of time, or at least lose its strength and rigidity. Also, corrugated board is subject to fire, mold, fungus and insect attack, and the like, and the industry has long sought an efficient means for incorporating additives into corrugated board which will counteract these attacks.
Accordingly, the industry has expended much time and effort to solve the problem. One approach to the solution of the problem of moisture attack has been to impregnate the linerboard and corrugated medium with a wax material to form a water barrier and thereafter fabricate the corrugated container-board. This approach has not been successful because the wax-impregnated linerboard and corrugating medium do not form satisfactory barriers against the encroachment of water vapor into the corrugated board. Therefore, the level of performance has not been all that is to be desired.
Another approach to a solution of this problem has been to form the corrugated board and then impregnate it with a wax. However, this approach has not been entirely satisfactory because water vapor is still able to penetrate the wax coating of the corrugated board. A still further approach has been to utilize a blend of wax-extendible copolymers as a coating for the respective elements of paperboard containers, as described in British Pat. No. 957,933.
Other attempts have also been made to produce corrugated board having enhanced rigid-when-wet or rigid-when-humid strength properties. These attempts have also been less than satisfactory. In general, these methods have involved resin impregnation of completed corrugated board or by impregnation with resin of the medium prior to corrugating and assembly of the corrugated board.
One method of resin-impregnation of finished corrugated board is disclosed in Arian et al. U.S. Pat. application, Ser. No. 54,201, filed July 13, 1970. In that method, liquid impregnating resins are poured into the open ends of finished corrugated boards and allowed to drain out the other ends. Another process for treating finished corrugated board is disclosed in U.S. Pat. No. 3,523,058.
Attempts have been made to employ in-line methods of resin treatment, such as Canadian Pat. No. 849,480, granted Aug. 18, 1970. These have not been satisfactory for commercial processing because of the difficulty in producing a uniformly resin-impregnated corrugated medium.
British Pat. No. 1,039,540 describes a process for coating materials with a foam produced from a coating material. However, the patent makes no suggestion as to how the problems of producing corrugated board having an additive incorporated into the board might be produced.
In general, if resin impregnation is performed prior to fluting the corrugated medium, the medium sticks to the corrugating rolls and board cracking is encountered during manufacture of the board. If resin impregnation is attempted after corrugation, it has not been possible to impregnate the board with any uniformity. Thus prior attempts at in-line resin impregnation have not provided uniform resin impregnation.